MARS - No these transmission do not originate from the red planet but come from the "Military Affiliated Radio System". There are literally 100's of these nets that are active OUTSIDE the normal amateur band limits. Most stations in these nets use a 3x1x3 type callsign, i.e. NNN9YES. Modes used are USB & SELCALL AMTOR. They typically handle message type traffic. DO NOT INTERFERE WITH THESE OPERATIONS! Since they are run by hams, you WILL get reported to the FCC for interference! Most of the nets I have heard typically only run during the day and most of them are finished by 2000 GMT. Unless you are into early broadcasting you probably will not have to worry about interfering with a MARS net but it helps to be aware that they are there and should be avoided at all costs!
SPANISH LANGUAGE NETS - Much speculation abounds as to the source and purpose of these operations. Since callsigns are never heard, it is probably safe to assume that these transmissions are of the unauthorized nature and will have little if any consequence for you by interfering with them. More than likely they will move to another frequency if they experience QRM from you. It's also possible that your signal could be QRM'd by these stations as well so avoidance is a good strategy.
MILITARY NETS - These nets can show up anywhere at anytime. Most originate from units on maneuvers in the field. Calls consist of 2 or 3 letters typically. Most military HF gear uses a channelized system so they usually have no idea where they are in the HF spectrum! I would not suggest that you operate on or near these types of nets. Not that it will get you reported to the FCC since most field units have no or difficult phone access, but that they will QRM your broadcasts. Avoidance again should be used. Take heart in knowing that these type of nets don't stay on any frequency for any great length of time.
RTTY - Stands for Radio TeleTYpe which is a forerunner of todays digital transmission modes and still are in widespread use on the HF bands. Avoid these stations! Not only are they licensed users but their signals will interfere with your signal much more than you would ever bother them. I have seen demonstrations of RTTY signals copied 100% when the signal, in my opinion, was totally useless! It is probably rare that your activities would generate a interference complaint from these stations.
SWBC - Super Wide BroadCast or ShortWave BroadCast, take your pick. These stations should be avoided like the plague! They typically run 100's of KILOWatts and ruin 10+Khz chunks of the spectrum where they broadcast. Your low power signal will be obliterated so give these station a wide berth!
OTHER SERVICES - Spy Numbers, Fishing Boats, VOLMET, Time. The shortwave spectrum is a busy place and you could run into anything! The basic rule of thumb is: if your not sure what it is just avoid it! This could keep you out of trouble and your listeners will appreciate it!